Water Testing for Swimming Pools

Testing the quality of the water in your pool is the most important step to ensure your pool is safe, clean and refreshing. Proper water balance will also extend the life of your pump, filter and any other equipment as well as making pool care quick and easy.

So, just what should be tested and why? First and most importantly, the sanitizer (chlorine, bromine etc.) needs to be within the proper range. Next pH and total alkalinity need to be monitored. When testing sanitizer the only useful measurement is for free chlorine or bromine; free means available for duty. Sanitizers are used up in keeping water clean as it combines with contaminants and forms chloramines. Chloramines are irritating, useless and need to be removed but form a component of Total Chlorine.

All water has pH that is a measurement of acidity or alkalinity. If pH is too low water will tend to be corrosive, especially to any metal like heaters or ladders. If pH is too high the water will tend to form scale inside the pool. To maintain the proper pH level of 7.2 to 7.8 Total Alkalinity needs to correct. Total Alkalinity measures dissolved carbonates in the water. These carbonates help buffer or “lock” pH at its proper level.

So testing for sanitizer, pH and Total Alkalinity is vital: what’s the best way to do it?

Glad you asked! There are three types of pool test kits commonly sold: OTO kits, DPD kits and Test Strips. The first two kits, OTO and DPD, are named for the reagent that tests the sanitizer level. These are the kits commonly portrayed with test tubes and eyedroppers. Adding reagents to the test water produces a color change that is compared to a chart and gives a value for the component being tested. Test strips also use color comparison but are dipped directly into the water.

OTO test kits have been around the longest and are common and cheap. OTO (OrthoTOlidene) reacts with chlorine and changes color. OTO kits also measure pH which is why they are often called Duo test kits. Unfortunately, OTO kits are pretty useless for measuring all-important free chlorine. The reaction that produces the color change is a two-stage reaction; the first change measures free chlorine and the second combined or total chlorine. The free chlorine reaction happens SOOOO fast that it’s virtually undetectable. In practice, an OTO kit is only reliable to measure pH!

DPD kits actually measure free chlorine as well as pH and alkalinity. They are a proven technology and recognized by most Health Departments as the acceptable way to test public pool water. There are drawbacks. Accuracy of test results is influenced by the freshness of the reagents, particularly DPD and the experience of the tester. DPD testing is also somewhat time consuming and complex but properly done the test is quite accurate.

Test strips are newer and truly a better way to go. Extremely fast and easy a strip is stuck in the water, removed and the reactive color changes compared to a chart right on the bottle. The basic three-way test strip measures free sanitizer, pH and total alkalinity at a glance. The results are not quite as specific as a DPD test but will definitely indicate whether the water is in the proper range or needs attention. The test is so easy that regular use is no pain at all. Test strips are now on the market will test everything from Baquacil to nitrate and salt levels.

The choice of test kit is, of course, up to each pool owner but if you want the right information throw out that Duo Test kit and switch to test strips or a DPD kit. You’ll spend less and enjoy the pool more ‘cause you know the score.